Touch sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touch object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
To an increasing extent, touch systems are designed to be able to detect two or more touches simultaneously, this capability often being referred to as “multi-touch” in the art.
There are numerous known techniques for providing multi-touch sensitivity, e.g. by using cameras to capture light scattered off the point(s) of touch on a touch panel, or by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into a touch panel.
U.S. Pat. No. 7,432,893 discloses a touch system that is operated by transmission of light inside a light transmissive panel. The touch system comprises two or more light sources that emit light into the panel such that the light propagates inside the panel with internal reflections and is detected by detectors around the periphery of the panel. An object touching the panel may frustrate the light whereby part of the light will leave its original propagation path and the detected signal will decrease. The location of the touching object may be computed using triangulation.
US 2009/0153519 also discloses a touch system that is operated by internal transmission of light. The touch system is defined as a “tomograph” comprising a light transmissive panel with flow-ports around the edges. Light is injected at one flow-port, propagates by internal reflection inside the panel and is transmitted from the panel at another flow-port, where a detector is arranged to measure the received light energy. An object touching the panel frustrates the light inside the panel and thereby decreases the transmitted light. The location of the touching object is computed by processing the received light energy at the flow-ports using well-known tomography methods.
WO 2010/064983 discloses an improved technique for processing of touch signals that represent detected light energy on a plurality of detection lines that extend across a touch surface of a light transmissive panel. The touch signals are processed into normalized signals that represent a ratio between light that is transmitted inside the panel when no objects are touching the panel and the light that is transmitted when there is one or more objects touching the panel.
WO 2011/139213 discloses an improved technique for tomographic reconstruction based on the touch signals from a touch system that is operated by transmission of light inside a light transmissive panel. The touch signals, which represent detected energy on a plurality of detection lines across the touch surface, are processed to generate a set of matched samples, which are indicative of estimated detected energy for fictitious detection lines that have a location on the surface portion that matches a standard geometry for tomographic reconstruction. This technique enables the touch system to be designed with any arrangement of detection lines across the panel, i.e. even arrangements that are mismatched to conventional tomographic techniques.
There remains a need to further improve the techniques of enabling detection of touching objects in touch systems that operate by frustration of light that is transmitted inside a light transmissive panel.